The modified Claus process has been widely applied for the production of sulfur from acid gas feeds containing hydrogen sulfide in admixture with varying amounts of carbon dioxide. The acid gas streams usually contain small amounts of hydrocarbons ranging from methane to butane and even hydrocarbons of higher molecular weight. All industrial Claus units start with a thermal reaction zone in which air is added in the stoichiometric quantity needed to react hydrogen sulfide to sulfur by the reaction: EQU H.sub.2 S+1/2O.sub.2 .fwdarw.S+H.sub.2 O
In the thermal reaction zone, sulfur dioxide is formed. A portion of the formed sulfur dioxide reacts with hydrogen sulfide to form additional sulfur. The main products of the thermal reaction zone are elemental sulfur, sulfur dioxide, unconverted hydrogen sulfide and a considerable amount of heat, which is ordinarily removed by generating steam in a heat exchanger. When hydrocarbons are present, there is also formed carbonyl sulfide and carbon disulfide by competing reactions. The gas from the thermal reaction zone is cooled and sulfur condensed and removed. The gas is reheated and passed to one or more catalytic stages where sulfur dioxide is reacted with hydrogen sulfide over alumina or bauxite catalyst to produce additional sulfur which is removed by cooling and condensation between catalytic stages. The catalytic sulfur forming reaction is: EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O
In the typical straight-through Claus process, as described above, if the inert content of the acid gas stream (e.g. CO.sub.2 or N.sub.2) exceeds about 50 percent by volume of the feed, the flame temperature becomes marginally low because of the inert burden. It then becomes necessary to change the flow diagram of the plant by either heating the air and/or the acid gas feed, or diverting part of the acid gas feed around the thermal reaction zone.
With increasing inert content, more and more of the acid gas feed must be diverted, and the thermal reaction zone approaches the situation in which sulfur dioxide is the major product of the thermal reaction zone, with little or no sulfur being formed and with little or no unreacted hydrogen sulfide in the flame. When the inert content of the acid gas reaches 75 to 80 percent, it becomes difficult or impossible to maintain a steady flame reaction, even when all the hydrogen sulfide is converted to sulfur dioxide.
A known method of dealing with a gas containing 80 percent or more inerts is disclosed in my U.S. Pat. No. 3,880,986, incorporated herein by reference. In this process, a thermal reaction is used to produce sulfur dioxide from elemental sulfur formed in the process.
In still other cases, a workable flame temperature may be sustained by adding hydrocarbon gas as a fuel. This, however, significantly complicates control of the process, creates the danger of forming tarry products and discolored sulfur, and reduces the recovery of sulfur by forming water, a reaction product which is adverse to the Claus equilibrium. It also amplifies the problem of forming carbonyl sulfide and carbon disulfide, which are difficult to convert on a continuous basis in the Claus plant.
As indicated, all of the industrially used Claus processes involve a thermal reaction step where sulfur dioxide is formed alone or with sulfur, the sulfur dioxide being later reacted with hydrogen sulfide to form sulfur. With no exceptions, the heat generated by the formation of sulfur dioxide is removed in a heat exchanger preceding the catalytic conversion stages. Thus, such plants all require a combustion chamber and a heat exchanger, which constitutes a substantial part of the cost of the entire plant and which adds considerably to the problems of controlling plant operation.
A need exists for an effective process for sulfur production which does not employ an expensive and difficult to control thermal reactor.